Gun mechanisms typically have a reciprocating "bolt" or similar part that causes the cartridge to be locked into the chamber of the barrel. Firearms also typically have a cartridge case that serves a dual purpose as a transportation container for the projectile, powder and primer during loading and as a gas seal during firing. Some mechanisms must be provided to cause the case to be removed from the firearm so that the next round can be loaded. This device is the ejector. In self-loading, magazine-fed firearms, it is also desirable that there be a mechanism or device that enables the operator to lock the bolt in the open position so that the firearm may be conveniently loaded.
In both operations, the mechanism should be simple and robust. Typically, in field use of a military firearm, jamming, as a result of moisture and dirt, can be avoided by minimizing the number of moving parts and by employing less complex parts.